Frequency mixers are often used in telecommunications as non-linear circuits or devices that accept two different input frequencies and present: 1) an output signal equal in frequency to the sum of the input frequencies, 2) a signal equal in frequency to the difference between the frequencies of the input signals, and if not filtered, 3) various undesired intermodulation products.
Frequency mixers are often implemented using discrete components to generate the nonlinearities useful to create the “mixing action.” Different prior art circuits have been used for frequency mixers, including diode bridges, Gilbert cell multipliers, log-ratio multipliers, and diode ring mixers. Both the diode bridge and Gilbert cell multiplier typically require a local oscillator (LO) signal to “flip” the polarity of a radio frequency (RF) input on a periodic basis. Typical LO signals are bipolar and of the form of a square or sinusoidal waveshape. When the LO is positive, the RF input passes to an intermediate frequency (IF) output without being sign reversed. When the LO is negative, the RF input is sign reversed as it passes to the IF output. As a result, the LO circuit “flips” the polarity of the RF signal, having the effect of multiplying by +1 or −1 (when practical circuit losses are neglected). The sign flipping induces the desired mixing action. Typical mixer prior art such as diode bridges and Gilbert cell designs have the general disadvantage of requiring high LO drive level (e.g., >0 dBm) that make them unattractive for low power applications (e.g., handheld battery operable equipment). They are also typically AC coupled on one or more ports so operation to low frequency (i.e., near DC) is limited by the coupling. In addition, the presence of coupling components prevents a monolithic realization and its attendant advantages (e.g., size, weight, and power reduction).
There are some prior art frequency mixers that are DC coupled on all its ports (e.g., LO, RF, and IF). One prior art example is FIG. 1, and it has been formed using three operational amplifiers, along with some support circuitry, as discrete components. The term operational amplifier is the common name for a circuit component known in engineering as a voltage feedback amplifier or voltage sense amplifier. The LO and RF ports are input into a first or input operational amplifier (OA1). A second operational amplifier (OA2) functions as an inverting, half-wave oscillator. A third or output operational amplifier (OA3) receives signals from the first and second operational amplifiers and functions as an inverting, summer operational amplifier.
The prior art example (FIG. 1) using the three operational amplifiers identified above has limited practical applications in RF frequency ranges. This frequency mixer (FIG. 1) using the three operational amplifiers as described above has been adequate for audio frequencies and some ultra-sonic applications (e.g., typically 20-100 KHz upper frequency), but it is inadequate for many RF applications.
For example, the diodes identified in the design are slow switching types due to low mobility carriers inherent in the part. Further, because of the discrete design methodology to realize the mixing function the diodes are not well matched, unlike what is possible on monolithic designs, so there can be significant “cross-over” distortion as the polarities are changed. Both these effects limit the upper end of the useful frequency range of this topology. An additional limitation of the frequency range is due to the performance limitations imposed by the operational amplifier (e.g., voltage feedback amplifier) topology used to implement the circuit shown in FIG. 1. The operational amplifier has well known design limitations pertaining to high frequency response. The limitations include output slew rate limitations, gain-bandwidth trade-off, and extreme sensitivity to load capacitance.
Another example of the limitations in the discrete approach is the matching and tracking required for the resistive elements. In discrete designs it is difficult to match resistive elements precisely and maintain the match over time and temperature. Monolithic circuits due to their size and structure on a common substrate provide a simple method to control the values of resistive elements at the time of manufacture, as well as time and ambient temperature variations. Violation of the match requirement induces well-known errors in the output such as common-mode signals, signal feed-thru, gain imbalance, and others.